The RI depends on the average electron density of the material. By adding GeO2 or TiO2 (or alkali oxide) to silica you are increasing its electron density, and therefore the RI. By adding F or B you decrease the electron density and the RI.

The structure of the glass therefore plays a secondary role, but if a change in structure (such as densification) results in a change in electron density, then the RI should also change.

Refractive index is directly proportional to electron density. Refractive Index of glasses will change when the structure is changed as it depends on the electron density.

RI can sometimes reflects the bonding energy or the bonding strength of he glass. Addition of heavy elements would increase the bonding strength thus the RI will also be increased. On the other hand, adding some halide elements would decrease the bonding strength thus the RI decreases.

Extending Soltani Toufik argument, we know that R.I. or Index of refraction, n=c/v.So it is dependent on wavelength (dispersion) which is a structure dependent property .Primarily, R.I. being the polarization and the propagation of the light phenomenon, the addition of any foreign substance which increases polarization(precisely the polarizability), in general, should cause an increase in RI as axplained below:

Suppose we add substances like TeO2, GeO2 into the ordinary silica glass containing extended O-Si-O network forming regular SiO4 tetrahedra which possess bridged oxygens[BO].Glass being condensed liquid is porous. The ions like Ti (IV), Ge (IV) enter the vacant sites. The oxygen having strong affinity for these metal ions would be attracted toward them to bring about distortion in a regular tetrahedral SiO4 in two steps. First it changes this regular tetrahedron to an asymmetric one by elongating three Si-O bonds and shortning one Si-O bond. Then, one Si-O breaks to form SiO3 [trigonal pyramid having non bridging oxygens (NBO)].This increase in NBO causes an increase in refractive index because NBO are more polarizeable than bridging oxygens (BO). The (F) normally would break whole of SiO4 structure (cf. SiF4) would decrease polarizability. B+3-O bond, itself, stronger than Si-O bond would hold B-O electron cloud all the more tightly without any change in polaziability.

In general, the refractive index of a glass increases with its density. However, there does not exist an overall linear relationship between the refractive index and the density for all silicate and borosilicate glasses. Refractive Index of glasses will change when the structure is changed as it depends on the electron density. RI can sometimes reflect the bonding energy or the bonding strength of the glass. Addition of heavy elements would increase the bonding strength thus the RI will also be increased. In general, the refractive index of a glass increases with its density. However, there does not exist an overall linear relationship between the refractive index and the density for all silicate and borosilicate glasses. In general, the refractive index of a glass increases with its density. However, there does not exist an overall linear relationship between the refractive index and the density for all silicate and borosilicate glasses. The refractive index of the core is higher than that of the cladding, so light in the core that strikes the boundary with the cladding at an angle shallower than critical angle will be reflected back into the core by total internal reflection.